AbstractWe describe the use of semiconductor nanomaterials, advanced fabrication methods and unusual device designs for a class of electronics capable of integration onto the inner and outer surfaces of thin, elastomeric sheets in closed-tube geometries, specially formed for mounting on the fingertips. Multifunctional systems of this type allow electrotactile stimulation with electrode arrays multiplexed using silicon nanomembrane (Si NM) diodes, high-sensitivity strain monitoring with Si NM gauges, and tactile sensing with elastomeric capacitors.

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Analytical calculations and finite element modeling of the mechanics quantitatively capture the key behaviors during fabrication/assembly, mounting and use. The results provide design guidelines that highlight the importance of the NM geometry in achieving the required mechanical properties. This type of technology could be used in applications ranging from human–machine interfaces to ‘instrumented’ surgical gloves and many others.

1. IntroductionElectrotactile stimulators and tactile sensors are of interest as bi-directional information links between a human operator and a virtual environment, in a way that could significantly expand function in touch-based interfaces to computer systems, with applications in simulated surgery, therapeutic devices, robotic manipulation, and others [1–5]. Electrotactile stimulation allows information to be presented through the skin, as an artificial sensation of touch, commonly perceived as a vibration or tingling feeling [6, 7]. Such responses are manifested through the excitation of cutaneous mechanoreceptors as a result of passage of a suitably modulated electrical current into the tissue [8]. Developed originally in the 1950s and further advanced in the 1970s, electrotactile stimulation has been traditionally explored for programmable braille readers and displays for the visually impaired as well as for balance control in individuals who suffer from vestibular disorders [5, 9–12]. Tactile sensors, on the other hand, measure the pressure created by physical contact, in a way that provides complementary information for potential use in feedback loops with the electrotactile process. Additional classes of sensors that can be important in this context include those for motion and temperature.

Incorporation of such technologies into a conformal, skin-like device capable of intimate, non-invasive mounting on the fingertips might, therefore, represent a useful achievement. Recent advances in flexible and stretchable electronics create opportunities to build this type of device [13–17]. Here we report materials, fabrication strategies and device designs for ultrathin, stretchable silicon-based electronics and sensors that can be mounted on the inner and outer surfaces of elastomeric closed-tube structures for integration directly on the fingertips. The active components and interconnects incorporate advanced mechanics designs, capable of accommodating large strains induced not only by natural deformations of the tubes during use, but also during a critical step in the fabrication process in which the tubes, specially formed to match the shapes of fingertips, are flipped inside out. This ‘flipping-over’ process allows devices initially mounted on the outer surface of the tube to be reversed to the inner surface, where they can press directly against the skin when mounted on the fingers. Analytical calculations and finite element modeling (FEM) provide quantitative insights into design layouts that avoid plastic deformation or fracture.

We demonstrate these concepts in multifunctional fingertip devices that include electrotactile electrode arrays multiplexed with Si nanomembrane (NM) diodes, strain sensors based on Si NM gauges, and tactile sensor arrays that use capacitors with low modulus, elastomeric dielectrics.